1. Field of the Disclosure
This disclosure relates to pulsed, broad-line fiber lasers. More particularly, the disclosure relates to pulsed broad-line fiber lasers for speckle reduction.
2. Prior Art
The advantages of using laser sources for the illumination of projection displays are compelling in comparison to using other light sources. They include: improved image brightness, improved power efficiency, improved image contrast, wider color gamut, reduced cost, reduced size of both the illumination source and the optical system, and improved depth of field for focus-free usage.
The main disadvantage of using laser sources is a potentially severe degradation of projected image quality due to the presence of a high contrast, high spatial frequency, granular pattern that seems to float at in front of the projected image plane. This pattern is known as speckle.
Since speckle arises due to the highly coherent nature of laser illumination, one means of improvement is to use sources with reduced coherence, such as the direct-emission green laser diodes. However, their linewidth is too narrow to reduce speckle to acceptable levels. Nor is the power of the most powerful laser diodes sufficient. The brightest and most power-efficient green lasers available today do not have sufficiently broad linewidth.
In recent years, the speckle suppression in laser projectors is often obtained by the use of a multimode fiber. The length of fiber should be long enough to achieve de-correlation of practically all fiber modes. The greatest advantage of this method is that it does not require mechanical movement. However, the correlation length of incident laser beam on screen increases by a factor equal to the magnification of the optical system. Therefore, to preserve the same speckle contrast on the screen as that at the distant fiber end, the method requires the number of multimode fibers be approximately equal to the square of the magnification of the projector objective lens (approximately equal to the number of pixels on the screen >300000). Despite significant efforts to develop a compact system for decreasing speckle noise to an acceptable for the human eye level, this problem still persists.
Hence there is a need for a fiber laser-illuminated projector system with reduced speckle noise that is of a simpler construction and more compact.